Condition responsive devices which provide an indication or control an operation in response thereto, are well-known in the prior art. Such devices have been used in many diverse environments. For example, it is known in the prior art to connect a pressure sensitive element, such as a Bourdon tube, to a source of external pressure, whereby in operation the tube is deflected by an amount proportional to sensed pressure variations. The mechanical deflection of the Bourdon tube is transduced into a mechanical or electrical output to provide an indication of the pressure in the tube or, alternatively, to control some other physical operation. An apparatus of this type is shown in U.S. Pat. No. 4,502,334 and in copending application Ser. No. 656,490 to Gray, filed Oct. 1, 1984.
Although such condition responsive devices have proved generally effective for indicating a condition or controlling an operation in response to a change in the condition, such devices may not be capable of providing the necessary degree of precision required in certain applications. For example, when a vane or other mechanical trigger device is attached to a Bourdon tube to facilitate the pressure measurement, this element increases the overall mass and friction of the Bourdon tube, thereby introducing error components into the output signal generated by the device. In precision applications, even a small degree of error in the output signal representative of the measured pressure can cause significant problems during the control process. Compensation for these error components often requires complex feedback control systems or circuits which increase the cost of the device and decrease reliability.
Accordingly, an advantage of the present invention is to provide a condition responsive apparatus having detection means associated with the pressure sensitive element which has only a minimum influence or otherwise affect the signal generated by the device.